The present invention relates to a navigational methods including updating navigational datas for aircraft and other aerospace vehicles; and more particularly the invention relates to navigation and acquisition of navigational update data for aircraft, the equipment including sensors, image processing and evaluating equipment and a reference store.
Navigational methods of the kind to which the invention pertains concerns an aircraft or a helicopter or any other flying object which scans the territory underneath by means of a sensor acquiring a scanning data that is compared with stored reference data. Moreover it is quite common practice that aircraft have several and, possibly, several different kinds of navigational systems.
The basic navigational system that is used is usually an inertia system which is supposed through satellite and other electromagnetic methods. Also, the pilot augments whatever information he has visually available through manual control. Generally speaking methods are known to compare acquired data which are to some extent characteristic of the territory over which the craft flies, with stored reference information. Known here are methods for image correlation; in particular an imaging sensor which usually is an infrared device and operates in the optical range, looks forward or straight down and takes pictures. These pictures will be stored temporarily as a pattern of many image points. The contrasting pattern of the imaged territory is then compared i.e. statistically correlated with reference patterns and from the sum total of the agreement or disagreement in the correlation one concludes on the position of the craft.
This known method as practiced has disadvantage that the reference pictures have to agree to a very high degree with the real time images just taken. However reflecting deviations resulting from different illumination, from seasonal variations (e.g. shown above) but also images taken on different times of the day interfere significantly with the method as the reference images may "look" different from the actual data acquired during the current overflight.
The known systems could indeed be improved by using infrared radiation rather than visible light and by actively illuminating the area under observation in order to obtain comparable conditions for evaluations, but the basic concept of requiring very accurate reference images and very accurate real time images on the current overflight for obtaining a good correlation is and remains still the same. One can only control to some extent the conditions to which the pictures are taken not to the method of correlation.
In a different class of methods known by terms such as TERCOM, TERPROM, SITAN, SPARTAN, PENETRATE correlation and navigational locating is carried out in the following manner. The territory of overflight is scanned by means of radar in order to obtain an elevational profile. This profile is then compared and correlated with reference strips supposedly having corresponding data of the same area in a strip length of several 100 m. The elevational profile is acquired here by simply measuring the effective altitude above a particular reference level. The strip of the elevational profile is digitally compared with the reference elevational data. This method is to a considerably extent independent from the deficiences outlined earlier but it is apparent that it is quite unsuitable for flight across flat areas and over territory with little elevational "signature". Depending on the kind of territory the acquired data or characteristic elevational changes may simply be insufficient for determining a sufficiently accurate trajectory and flight path.